Recent increase in the use of fiber (or fiber tow) reinforced polymer matrix composite in aerospace, automotive, wind energy turbine blades, offshore drilling. sports equipment and other structures has motivated the development of new composites having increased strength and increased specific strength (strength per unit mass of composites). Composites fail in three different modes: matrix cracking, fiber fracture, and debonding at the interface. Load transfer has to be by way of the interface between fibers and matrix. Enhancing matrix stiffness and the strength of the matrix surrounding a fiber, and increasing the fiber/matrix interfacial strength will increase the stiffness and strength of the overall composites.
Load transfer has to take place through the interface between the fiber and polymer matrix, and the matrix is responsible for carrying the shear loads. The stiffness and transverse strength of fiber-reinforced composites depends on the mechanical behavior at the interface with a thickness approximately 100 nm or less. Swadener et al. determined that the failure or the delamination of a glass fiber occurs in the matrix 3 nm away from the fiber surface. [Swadener 1999]. Similar behavior has also been observed in single walled carbon nanotube (SWNT) nanocomposites. Ding et al. observed that a few nanometers of polycarbonate remains wrapped around a SWNT when the SWNT is pulled out of the polycarbonate matrix in fracture. [Ding 2003]. In order to increase the strength of composites, it is critical to improve the interfacial mechanical properties through modification of the polymer matrix, fibers or the interface.
Huang et al. has reported the interfacial micromechanics of carbon fibers in thermoplastic by determining the distribution of interfacial shear stress along fibers in single-fiber model composites using Raman spectroscopy. [Huang 1996]. The variations of fiber strain with position along the fiber in these composites are almost linear, indicating that stress transfer from matrix to fiber in the system is predominantly by frictional shear. It was found that the maximum values of interfacial shear strength for the polymethyl methacrylate (PMMA) and polycarbonate (PC) model composites are much lower than the value obtained for the same fibers in a thermosetting epoxy resin matrix. These low values of interfacial shear stress in thermoplastic systems can be explained by the lack of chemical bonding between the fiber and matrices, and possibly the effect of residual solvent. The interfacial adhesion in the systems stems primarily from mechanical interlocking, which can be enhanced by preparing the composites at higher temperatures. It is shown for PMMA that the maximum interfacial shear stress correlates very well with the radial pressure on the fiber as a result of thermal mismatch between the fiber and matrix.
In recent years, considerable effort has been made to enhance interfacial shear strength using CNTs grafted onto glass or carbon fibers to increase the interfacial shear strength (IFSS) [Mei 2010; Thostenson 2002; Qian 2010 I; Qian 2010 II; F Zhang 2009; Q Zhang 2009; Zeng 2010; Zhao 2010]. Besides nanotubes, grafting polyhedral oligomeric silsesquioxanes (POSS), an emerging new chemical technology for nano-reinforced organic-inorganic hybrids, has been demonstrated by Zhao et al. [Zhao 2010] and a 61% increase in Interfacial Shear Strength (IFSS) is claimed. Sager et al. showed improvement in interfacial shear strength with CNTs coated carbon fiber embedded in epoxy matrix by a single fiber fragmentation method. [Sager 2009]. Two configurations have been investigated: carbon fiber having radially (with 11% increase in IFSS) and randomly (with 71% increase IFSS) aligned multiwalled nanotube (MWNTs) embedded in epoxy. The use of randomly oriented MWNTs is observed to give higher interfacial shear strength due to a potentially higher percentage of MWNTs aligned with the ±45° principal stress directions under pure shear loading. However, they have reported significant reduction in ultimate tensile strength and modulus for the composites. On the other hand, Sharma et al. have demonstrated that growing CNTs on carbon fiber provides 69% improved tensile strength compared with that for untreated carbon fibers. [Sharma 2011]. Bekyarova et al. has demonstrated selective deposition of multi- and single walled carbon nanotubes (CNTs) on woven carbon fabric by an electrophoresis technique. [Bekyarova 2007 I]. The introduction of 0.25% of MWNTs in the carbon fiber (CF)/epoxy composites results in an enhancement of the interlaminar shear strength by 27% and significantly improved out-of-plane electrical conductivity. Reports on modification of the carbon fiber with surface treatment alone to increase the IFSS are also claimed. [He 2010; Li 2008; Moon 1992].
Besides engineering the carbon fiber to enhance the fiber/matrix interface, dispersing regular and functionalized CNTs in epoxy resin is another approach to achieve improvement in IFSS. [Zhu 2012; Zhu 2007; Zhu 2003; Che 2009; Ma 2009; Rubi 2011; Sui 2009]. Carbon nanotube fiber itself has also been used to evaluate the IFSS by other groups. [Ganesay 2011; Özden-Yenigün 2012; Zu 2012]. Bekyarova et al. have demonstrated dispersion of SWNT-COOH in epoxy and subsequent use for infiltration of carbon fabric (CF) by the vacuum assisted resin transfer molding technique to fabricate SWNT-COOH/epoxy/CF composites. [Bekyarova 2007 II]. Mechanical tests demonstrate that the incorporation of SWNT-COOH improves the mechanical performance of the composites and produces a 40% enhancement of the shear strength for a SWNT-COOH loading of 0.5 wt %. Tseng et al. and Cheng et al. have shown that functionalizing CNTs by plasma modification improves the tensile strength and electrical conductivity of covalently-integrated epoxy composites. [Tseng 2007; Cheng 2010]. Another approach for improving IFSS is to modify the interlaminar interface, which was used by Fan et al. and Tsotsis to fabricate the hybrid MWNT/glass/epoxy composites. [Fan 2008; Tsotsis 2009]. Up to 33% increase in the IFSS is reported by the introduction of MWNT into the composite. Multifunctional performance of carbon nanotube offering improvement in electrical and thermal conductivity by dispersing CNTs in thermoset and thermoplastic resin has been reported by many groups. [Assael 2009; Cheng 2010”; Kotaki 2009].
Godara et al. has compared the gain in IFSS by introducing carbon nanotube in unidirectional glass fiber/epoxy macro-composites in three ways: (1) in the fiber sizing, (2) in the matrix and (3) in the fiber sizing and matrix simultaneously. Interfacial shear strength was investigated using single-fiber push-out microindentation. [Godara 2010; Godara 2009]. The results of the test reveal an increase of IFSS in all three cases. The same group (Godara et al.) has demonstrated the influence of dispersed CNTs in epoxy matrix on the coefficient of thermal expansion (CTE) for various composites measured in the transverse direction to the fiber orientation. [Godara 2010; Godara 2009]. They have reported dispersing thin-MWNTs and functionalized (with amine group, —NH2) double walled nanotubes (DWNTs) lowers the CTE most effectively compared to that of MWNTs. This is possibly because thin-MWNTs and functionalized DWNTs effectively block thermally induced movements of the chains, due to their reduced size and higher interaction, thereby significantly reducing the increase in free volume. The functionalized DWNTs are even more effective due to the alignment of the polymer chains along the CNTs in axial direction because of the presence of surface —NH2 functional groups. This would result in a reduction in average free polymer chain length and association of part of the polymer chains with CNT having near-zero coefficient of thermal expansion (CTE), leading to a significant decrease in the CTE. Barber et al. have reported how the interfacial strength between a single CNTs and a polymer matrix increases dramatically when the nanotube surface is chemically modified, though the data scatter was very high. [Barber 2006; Barber 2003]. The tests have been conducted by pulling out single CNTs using an atomic force microscopy (AFM) tip. A comprehensive computational model has been developed for fiber pull-out test by Zhong et al. [Zhing 2003].
Advances in characterization of the IFSS have also been reported by some group on single fiber pull test conducted in-situ in a scanning electron microscope (SEM). Manoharan 2009. Desaeger et al. have demonstrated micro-indentation tests to evaluate IFSS on different kinds of reinforced polymer composites (carbon and glass fibers embedded in thermoplastic and thermoset matrices). [Desaeger 1993] Besides fiber reinforced polymer composites, metal and carbon[43] matrix composites are also being investigated for interfacial properties using nanoindentation technique [Ureña 2005; Tezcan 2008].
Despite these advances, major performance improvements are needed to address the increasing practical demands for light-weight, ultra-strong, ultra-high-modulus composites.